1. Field of the Invention
The present invention is generally related to a method of coding artefacts reduction, in particular of a discrete decoded picture.
Generally, picture coding/compression standards like JPEG or MPEG are based on block-based discrete cosine transformation (DCT) encoding. Therefore, a loss of quality might arise within an encoded image, such as blocking or ringing artefacts.
2. Description of Related Art
Different methods are proposed to reduce such a loss of quality, like from H. W. Park, Y. L Lee “A Postprocessing Method For Reducing Quantization Effects And Low Bit-Rate Moving Picture Coding”, IEEE Transactions on Circuit Systems For Video Technology, Vol. 9, No. 1, February 1999, pages 161–171, and by S. Minami, A. Zakhor “An Optimization Approach for Removing Blocking Effects in Tranform Coding”, IEEE Transactions on Circuits and Systems for Video Technology, Vol 5, No. 2, April 1995, pages 74 to 82.
In particular, according to the first mentioned described postprocessing method blocking artefacts are reduced by one-dimensional horizontal and vertical lowpass filtering and the ringing noise is reduced by two-dimensional signal-adaptive filtering. Since such restoration techniques for removing blocking effects are described to result in unnecessary blurring of the image in the latter mentioned paper, a new approach is proposed therein which exploits the correlation between the intensy values of boundary pixels of two neighbouring blocks. This approach is based on the theoretical and empirical observation that under mild assumptions quantization of the DCT coefficients of two neighbouring blocks increases the expected value of the mean squared difference of slope (MSDS) between the slope across two adjacent blocks, and the average between the boundary slopes of each of the two blocks. Since the amount of this increase is dependent upon the width of quantization intervals of the transformed coefficients among all permissible inverse quantized coefficients the set of DCT coefficients which reduces the expected value of this MSDS by an appropriate amount is most likely to decrease the blocking effect.
U.S. Pat. No. 5,442,462 shows an apparatus and a method for smoothing images applicable for adaptive spatial noise reduction, adaptive temporal noise reduction and a combination thereof. It is described to use the teaching to cope with noise artifacts resulting from spatial and spatio-temporal image coding and compression methods such as block transform techniques. Further, iterated computation in presence of excessive noise is allowed. The adaptive smoothing technique determines a most suitable weighting for each processed pixel. Furthermore, also a manual input from a user is described to adapt the output image to the desires of the user. Finally it is described that the disclosed method performs the time domain smoothing after the spatial smoothing operation is performed.
WO 99/22509 describes an image data post-processing method for reducing quantization effect and an apparatus therefore. In the introductory part of the specification a spatio-temporal adaptive post-filtering is mentioned for low bit rate video signals. Further, according to the proposed method qualified semaphores are generated using the distribution of inverse quantization coefficients of inverse-quantized image data and a motion vector, whereafter the reducing of blocking artifacs is strongly or weakly performed depending on the generated blocking semaphore (similar for ringing). Although a temporal filtering is not directly described, the proposed method reduces the quantization effect adaptively by using spatial frequency and temporal information.
However, all of the above described methods inherit the disadvantage of a relatively high needed calculation power because of processing an entire image and in particular to determine a filtering characteristic in the aim to reduce coding artefacts based upon determination of an objective parameter influencing the respective filtering. Further, the image improvement is only noticable by encoding with a relatively low bit rate. That means that the improvement topples to degradation applying the method to encode sequences with high image quality.
Therefore, it is the object underlying the present invention to provide a further method to reduce coding artefacts within a discrete decoded picture which overcomes the above mentioned problems of the prior art methods.